1. Field of the Invention
The present invention relates generally to microwave oscillators. More particularly, the invention relates to a Gunn diode oscillator employing subharmonic locking by signal injection using a diode bridge. The diode bridge provides a feedback signal indicative of the phase of the cavity oscillation which can be used to provide a phase-locked oscillation.
2. Description of Related Art
In many microwave applications, solid state microwave oscillators are now being used. Many such oscillators use diodes, such as tunnel diodes, avalanche diodes and Gunn diodes, to generate the microwave signal. Typically, the diode is electromagnetically coupled to a tuned or resonant cavity. Tunnel diodes and avalanche diodes are junction semiconductors. They have one or more doped regions with junctions in between. In contrast, the Gunn diode is a bulk semiconductor without differently doped regions defining junctions. In general, all of these diodes are negative resistance devices. They can be used to generate microwave oscillations by coupling to a tuned cavity of high Q. Usually, the tuned cavity offers a positive resistance which is designed to cancel out the negative resistance of the diode. The impedance of the cavity is constructed to be the conjugate match of the diode reactance. By energizing the diode, microwave oscillations are set up in the cavity. The cavity may be coupled via an output port to a waveguide or the like, thereby affording a microwave oscillator which can be used in a wide variety of communications and radar applications, to name but two. Although junction semiconductors and bulk semiconductors can both be used, in general, bulk semiconductors are capable of higher power output and can be made to operate at higher frequencies.
In designing a microwave system, it is often desirable to have control over the frequency of the oscillation. In a multiple channel microwave transmitter, for example, it is desirable to be able to generate or synthesize a plurality of different channels or frequencies by shifting the oscillation frequency of the microwave oscillator. Some fairly complex circuits have been devised to accomplish this in conventional systems. Also, in many instances, it is highly desirable to have to microwave oscillations phase-locked to a more readily controlled low-frequency reference signal. However, conventional microwave oscillators have had great difficulty in providing a phase-locked signal.
In providing controlled microwave oscillations, it is known to inject a fundamental signal into the resonant cavity using either reflection injection with a three port circulator or direct injection with a probe, causing the frequency of the cavity oscillation to frequency lock or track with the injected signal. By controlling the frequency of the injected signal, the frequency of the microwave oscillation can be controlled. While providing a frequency-locked signal, this conventional approach does not provide a phase-locked signal. Moreover, the electronic circuit for developing the subharmonic injection requires a parasitic oscillating signal and an idler signal and is quite complex and expensive. In addition, when injecting the signal in this fashion, a plurality of multiples of the subharmonic signal will also be injected unless filtered out. This adds further to the complexity and expense in developing a workable microwave system.
In order to provide a phase-locked performance in a conventional system, a complex closed-loop sampling system is usually employed. Aside from adding considerably to the circuit complexity, sampling systems of this nature require several multipliers and usually some form of phase detector and integrator in the closed loop. Circuits of this type are traditionally quite slow in responding to channel or frequency changes. Hence, there can be a considerable delay before phase lock is achieved after a channel selection has been made.